Image processing device, image processing method, and program

ABSTRACT

A frame memory stores a plurality of captured images. An image selection unit selects a reference image, which is a standard for composition, from images other than an image which is captured earliest and an image which is captured latest among the plurality of images stored in the frame memory and selects images to be combined with the reference image from a group of images which are captured earlier and later than the reference image among the plurality of images. A composition processing unit composes the images to be combined selected by the image selection unit with the reference image selected by the image selection unit to generate a composite image.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing device, an image processing method, and a program.

Priority is claimed on Japanese Patent Application No. 2013-172634, filed Aug. 22, 2013, the contents of which are incorporated herein by reference.

2. Description of Related Art

A method has been known which sets the image that is captured earliest as a reference image and includes using only images that are captured later than the reference image, or which sets the image that is captured latest to a reference image and includes using only images that are captured earlier than the reference image, among a plurality of images stored in a frame memory in a process of composing a plurality of images to remove noise in the time direction (for example, see Japanese Unexamined Patent Application, First Publication No. 2012-186593).

SUMMARY OF THE INVENTION

According to a first aspect of the invention, an image processing device includes: a memory that stores a plurality of captured images; an image selection unit that selects a reference image, which is a standard for composition, from images other than an image which is captured earliest and an image which is captured latest among the plurality of images stored in the memory and selects images to be combined with the reference image from a group of images which are captured earlier and later than the reference image among the plurality of images; and a composition processing unit that composes the images to be combined selected by the image selection unit with the reference image selected by the image selection unit to generate a composite image.

According to a second aspect, in the image processing device according to the first aspect, the image selection unit may determine whether images which are captured at times close to the capture time of the reference image among the plurality of images stored in the memory is used as the images to be combined and select, as the images to be combined, the image which is determined to be used as the images to be combined.

According to a third aspect, in the image processing device according to the second aspect, the image selection unit may select, as the reference image, an image which is captured at a time closest to an intermediate time between a time when the oldest image is captured and a time when the newest image is captured among the plurality of images stored in the memory.

According to a fourth aspect, in the image processing device according to the second aspect, the image selection unit may select the reference image such that the number of images which are captured earlier than the reference image is closest to the number of images which are captured later than the reference image, among the plurality of images stored in the memory.

According to a fifth aspect, the image processing device according to the second aspect may further include a movement amount estimation unit that estimates an amount of movement of an object in the image. The image selection unit may select the reference image on the basis of the amount of movement estimated by the movement amount estimation unit.

According to a sixth aspect, in the image processing device according to the fifth aspect, the movement amount estimation unit may compare a plurality of images to estimate the amount of movement of the object in the plurality of images.

According to a seventh aspect, in the image processing device according to the fifth aspect, the movement amount estimation unit may detect the amount of movement of an imaging element that captures the plurality of images to estimate the amount of movement of the object in the plurality of images.

According to an eighth aspect, in the image processing device according to the fifth aspect, the image selection unit may select the reference image from a plurality of images in which the estimated amount of movement is equal to or less than a predetermined threshold value.

According to a ninth aspect, in the image processing device according to the fifth aspect, the image selection unit may exclude, from candidates of the reference image, the image which is captured earliest and the image which is captured latest among a plurality of images in which the estimated amount of movement is equal to or less than a predetermined threshold value.

According to a tenth aspect, in the image processing device according to the fifth aspect, the image selection unit may select the reference image from an image section including a predetermined number of images which are consecutive in order of the capture time and are less than the number of images stored in the memory. In the image section, an integrated value of the estimated amount of movement in a plurality of images forming the image section may be the minimum.

According to an eleventh aspect, in the image processing device according to the tenth aspect, the image selection unit may exclude, from candidates of the reference image, the image which is captured earliest and the image which is captured latest among the plurality of images forming the image section.

According to a twelfth aspect, in the image processing device according to any one of the fifth to eleventh aspects, the image selection unit may select an image in which the estimated amount of movement is the minimum as the reference image.

According to a thirteenth aspect, in the image processing device according to the second aspect, the image selection unit may end the process of determining the images to be combined when it is determined that a predetermined number of consecutive images are not used as the images to be combined.

According to a fourteenth aspect, the image processing device according to any one of the first to thirteenth aspects may further include a noise amount estimation unit that estimates an amount of noise in the plurality of images stored in the memory. The image selection unit may determine the number of images required for composition, which is the number of images required to generate the composite image, on the basis of the amount of noise estimated by the noise amount estimation unit.

According to a fifteenth aspect, in the image processing device according to the fourteenth aspect, the image selection unit may count the sum of the number of selected reference images and the number of selected images to be combined and end the process of determining the images to be combined when the count result reaches the number of images required for composition.

According to a sixteenth aspect, in the image processing device according to any one of the first to thirteenth aspect, the image selection unit may predetermine the number of images to be combined, determine whether the number of selected images to be combined reaches the predetermined number of images to be combined, and end the process of determining the images to be combined when the number of selected images to be combined reaches the predetermined number of images to be combined.

According to a seventeenth aspect, in the image processing device according to the fifth aspect, the image selection unit may select the images to be combined from a plurality of images in which the estimated amount of movement is equal to or less than a predetermined threshold value.

According to an eighteenth aspect, there is provided an image processing method that composes a plurality of images stored in a memory to generate a composite image. The image processing method includes: selecting a reference image, which is a standard for composition, from images other than an image which is captured earliest and an image which is captured latest among the plurality of images stored in the memory; selecting a images to be combined with the reference image from a group of images which are captured earlier and later than the reference image among the plurality of images; and composing the selected images to be combined with the selected reference image to generate the composite image.

According to a nineteenth aspect, in the image processing method according to the eighteenth aspect, in the selection of the images to be combined, it may be determined whether images which are captured at times close to the capture time of the reference image among the plurality of images stored in the memory is used as the images to be combined, and the image which is determined to be used as the images to be combined may be selected as the images to be combined.

According to a twentieth aspect, in the image processing method according to the nineteenth aspect, in the selection of the reference image, among the plurality of images stored in the memory, an image which is captured at a time closest to an intermediate time between a time when the oldest image is captured and a time when the newest image is captured may be selected as the reference image.

According to a twenty-first aspect, in the image processing method according to the nineteenth aspect, in the selection of the reference image, the reference image may be selected such that the number of images which are captured earlier than the reference image is closest to the number of images which are captured later than the reference image, among the plurality of images stored in the memory.

According to a twenty-second aspect, the image processing method according to the nineteenth aspect may further include comparing a main image with another image to estimate an amount of movement of an object in the main image. In the selection of the reference image, the reference image may be selected on the basis of the estimated amount of movement.

According to a twenty-third aspect, in the image processing method according to the twenty-second aspect, in the selection of the reference image, an image in which the estimated amount of movement is the minimum may be selected as the reference image.

According to a twenty-fourth aspect, in the image processing method according to the twenty-second aspect, in the selection of the images to be combined, the images to be combined may be selected from a plurality of images in which the estimated amount of movement is equal to or less than a predetermined threshold value.

According to a twenty-fifth aspect, there is provided a computer-readable device storing a program for generating a composite image that puts together a plurality of images stored in a memory. The program causes a computer to perform steps of:

selecting a reference image, which is a standard for composition, from images other than an image which is captured earliest and an image which is captured latest among the plurality of images stored in the memory; selecting a images to be combined with the reference image from a group of images which are captured earlier and later than the reference image among the plurality of images; and composing the selected images to be combined with the selected reference image to generate the composite image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the structure of an image processing device according to a first embodiment.

FIG. 2 is a schematic diagram illustrating an example of frames stored in a frame memory in the first embodiment.

FIG. 3 is a schematic diagram illustrating the relationship between the time when an imaging element captures an image and the position of the imaging element in the first embodiment.

FIG. 4A is a schematic diagram illustrating the procedure of a process according to the related art which composes a frame F₀ and a frame F_(N) to generate a composite image.

FIG. 4B is a schematic diagram illustrating the procedure of the process according to the related art which composes the frame F₀ and the frame F_(N) to generate the composite image.

FIG. 4C is a schematic diagram illustrating the procedure of the process according to the related art which composes the frame F₀ and the frame F_(N) to generate the composite image.

FIG. 5 is a graph illustrating the allowable amount of movement X of the imaging element when the frame F₀ which is captured at a time t₀ is used as a reference image in the first embodiment.

FIG. 6 is a graph illustrating the allowable amount of movement X of the imaging element when a frame F_(N/2) which is captured at a time t_(N/2) is used as the reference image in the first embodiment.

FIG. 7 is a flowchart illustrating a composite image generation process of the image processing device according to the first embodiment.

FIG. 8 is a block diagram illustrating the structure of an image processing device according to a second embodiment.

FIG. 9 is a flowchart illustrating a composite image generation process of the image processing device according to the second embodiment.

FIG. 10 is a schematic diagram illustrating an example of the movement of an imaging element in the second embodiment.

FIG. 11 is a graph illustrating an example of the relationship between a time t and the amount of movement of the imaging element in the second embodiment.

FIG. 12 is a schematic diagram illustrating an example of the movement of the imaging element in the second embodiment.

FIG. 13 is a graph illustrating an example of the relationship between the time t and the amount of movement of the imaging element in the second embodiment.

FIG. 14 is a block diagram illustrating the structure of an image processing device according to a third embodiment.

FIG. 15 is a flowchart illustrating a composite image generation process of the image processing device according to the third embodiment.

FIG. 16 is a block diagram illustrating the structure of an image processing device according to a fourth embodiment.

FIG. 17 is a flowchart illustrating a composite image generation process of the image processing device according to the fourth embodiment.

FIG. 18 is a graph illustrating an example of the relationship between a time t and the amount of movement of an imaging element in each frame stored in a frame memory in the fourth embodiment.

FIG. 19 is a graph illustrating an example of the relationship between the time t and the amount of movement of the imaging element in the fourth embodiment.

FIG. 20 is a schematic diagram illustrating an example of the selection of a reference image in the fourth embodiment.

FIG. 21 is a block diagram illustrating the structure of an endoscope apparatus according to a fifth embodiment.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

Hereinafter, a first embodiment of the invention will be described with reference to the drawings. FIG. 1 is a block diagram illustrating the structure of an image processing device 10 according to this embodiment. In FIG. 1, the image processing device 10 includes a composition control unit 11 and a frame memory (memory) 12. The composition control unit 11 includes an image selection unit 13 and a composition processing unit 14 and generates a composite image. The frame memory 12 stores input video signals (a plurality of captured images). The video signal is, for example, a moving image and includes frames which are continuously captured.

When a composition instruction is input, the image selection unit 13 selects a reference image, which is a standard for composition, and a plurality of images to be combined (to be composed) with the reference image from the frames included in the video signal which is stored in the frame memory 12. The composition processing unit 14 composes the reference image selected by the image selection unit 13 with the images to be combined to generate a composite image. The composite image generated by the composition processing unit 14 is an image obtained by removing noise in a time direction from the reference image. That is, the composition processing unit 14 composes the images to be combined with the reference image to remove noise from the reference image.

Next, an example of the frames stored in the frame memory 12 will be described. FIG. 2 is a schematic diagram illustrating an example of the frames stored in the frame memory 12 in this embodiment. In FIG. 2, the frame memory 12 stores N frames F₀, F₁, F₂, . . . , F_(M−1), F_(M), F_(M+1), . . . , F_(N−2), F_(N−1), F_(N) with different capture times t (N is a natural number and M is an integer that is equal to or greater than 0 and equal to or less than N). In addition, in FIG. 2, the frames are captured in the order of the frame F₀ to the frame F_(N), the frame F₀ is captured earliest, and the frame F_(N) is captured latest. For example, the frame F₀ is captured at t₀ and the frame F₁ is captured at t₁. The other frames F₂ to F_(N) are captured at t₂ to t_(N), respectively.

Next, the video signal input to the frame memory 12 will be described. For example, a video signal which is captured by an imaging element provided at the tip of an endoscope apparatus is input to the frame memory 12. FIG. 3 is a schematic diagram illustrating the relationship between the time when an imaging element 104 provided at the tip of the endoscope apparatus captures an image and the position of the imaging element 104. In FIG. 3, the imaging element 104 captures the image of an object for the period from the time t₀ to the time t_(N). Then, the imaging element 104 outputs the frame F₀ at the time t₀ and outputs the frame F_(N) at the time T_(N) (see FIG. 2). In addition, the position of the imaging element 104 is different at the time t₀ and the time t_(N). In FIG. 3, the amount of movement X of the imaging element 104 between the imaging element 104 at the time t₀ and the imaging element 104 at the time t_(N) is X_(N).

When the object remains stationary, the position of the object in the frame F₀ is different from the position of the object in the frame F_(N) since the imaging element 104 is being moved. Therefore, when the frame F₀ and the frame F_(N) are composed with each other to generate a composite image, a composition process needs to be performed after positioning is performed such that the position of the object in the frame F₀ and the position of the object in the frame F_(N) are aligned with each other.

FIGS. 4A to 4C are schematic diagrams illustrating the procedure of the known process of composing the frame F₀ and the frame F_(N) to generate a composite image. In FIGS. 4A to 4C, the frame F₀ output from the imaging element 104 shown in FIG. 3 is the reference image and the frame F_(N) is the images to be combined (see the frames shown in FIG. 2). In addition, a letter “A” is captured as the object. The frame F₀ and the frame F_(N) each include noise.

FIG. 4A is a schematic diagram illustrating a superimposed image of the frame F₀ and the frame F_(N). The position of the imaging element 104 is different at the time t₀ when the frame F₀ is captured and at the time t_(N) when the frame F_(N) is captured. Therefore, in FIG. 4A, the deviation between the position of an object “A” 401 in the frame F₀ and the position of an object “A” 402 in the frame F_(N) is the amount of movement X_(N). The positional deviation between the object “A” in the images to be combined (frame F_(N)) and the object “A” in the reference image (frame F₀) is referred to as the amount of blur.

FIG. 4B is a schematic diagram illustrating two frames, with the position of the object “A” in the frame F_(N) being aligned with the position of the object “A” in the frame F₀. In FIG. 4A, the composition processing unit 14 aligns the position of the object “A” 402 in the frame F_(N), which is the images to be combined, with the position of the object “A” 401 in the frame F₀ that is the reference image. As such, when the object is positioned in order to move the position of the frame F_(N), a region in which the frame F₀ and the frame F_(N) overlap each other and a region in which the frame F₀ and the frame F_(N) do not overlap each other are generated.

FIG. 4C is a schematic diagram illustrating a composite image 500 generated by calculating the weighted average of the frame F₀ and the frame F_(N) which are positioned relative to each other. The region of the composite image 500 is the same as the region of the frame F₀ which is the reference image. In FIG. 4C, noise is reduced in the region in which the frame F₀ and the frame F_(N) overlap each other. However, since the composite image 500 is generated by calculating the weighted average of the frame F₀ and the frame F_(N), the brightness of a region 501 in which the frame F₀ and the frame F_(N) do not overlap each other is low.

Therefore, the maximum amount of blur of the images to be combined is determined according to the allowable region 501. For example, when the size of the allowable region 501 is widened, because the images to be combined with a large amount of blur are used to generate the composite image 500, it is possible to use a large number of frames. Accordingly, although it is possible to improve a noise reduction effect, the effective size of the image without reduced brightness is small. When the allowable region 501 is narrowed, because it is difficult to use the images to be combined with a large amount of blur to generate the composite image 500, the composite image is small. Consequently, the noise reduction effect is reduced, but the effective size without reduced brightness is large. In accordance with these things, the maximum amount of blur is decided. When the amount of movement X_(N) of the imaging element 104 is large, the amount of blur of the images to be combined is large. Therefore, the allowable amount of movement X_(N) of the imaging element 104 is determined on the basis of the allowable range of the region 501.

Next, the relationship among the reference image, the allowable amount of movement X per unit time of the imaging element 104, and the allowable range which is selected as the images to be combined will be described.

FIG. 5 is a graph illustrating the allowable amount of movement X per unit time of the imaging element 104 when the oldest frame F₀ which is captured at the time t₀ among a plurality of frames stored in the frame memory 12 is set as the reference image. In the graph of FIG. 5, the vertical axis indicates the amount of movement per unit time of the imaging element 104 and when the interval between captured frames is constant, the amount of movement per unit time of the imaging element 104 is regarded as the amount of movement per between frames. Hereinafter, the allowable amount of movement X per unit time of the imaging element 104 is simply referred to as the amount of movement X. The horizontal axis of the graph of FIG. 5 indicates a time t. In addition, the imaging element 104 moves in a straight line with a constant speed. The allowable amount of movement X of the imaging element 104 is referred to as X_(a).

In FIG. 5, the amount of movement X of the imaging element 104 is more than the amount of movement X_(a) at a time after a past time t_(M). Therefore, the frames F which can be used as the images to be combined to generate the composite image 500 are the frames F₀ to F_(M) which are captured for the period from the time t₀ to the time t_(M) (see FIG. 2). That is, when the frame F₀ is set as the reference image, it is difficult to use the frames F_(M+1) to F_(N) which are captured at the times after the time t_(M) as the images to be combined.

FIG. 6 is a graph illustrating the allowable amount of movement X of the imaging element 104 when the frame F_(N/2) which is captured at a time t_(N/2) is used as the reference image in this embodiment. In the graph of FIG. 6, the vertical axis, the horizontal axis, and the allowable amount of movement X_(a) of the imaging element 104 are the same as those in the graph of FIG. 5.

In this embodiment, as the reference image, a frame is selected from the frames other than the oldest frame F₀ which is captured at the time t₀ and the newest frame F_(N) which is captured at the time t_(N) in the frame memory 12. Specifically, a frame F_(N/2) which is captured at a time t_(N/2) closest to an intermediate (middle) time between the time when the frame F₀ is captured and the time when the frame F_(N) is captured is selected as the reference image.

In FIG. 6, when the frame F_(N/2) captured at the time t_(N/2) is set to the reference image, both the position of the imaging element 104 at the time t_(N/2) when the reference image is captured and the amount of movement X based on the position of the imaging element 104 at the time t₀ are not more than the amount of movement X_(a). Both the position of the imaging element 104 at the time t_(N/2) when the reference image is captured and the amount of movement X based on the position of the imaging element 104 at the time t_(N) are not more than the amount of movement X_(a). Therefore, when the composite image 500 is generated, all of the frames before the frame F_(N/2), which is the reference image, and all of the frames after the frame F_(N/2), which is the reference image, can be used as the images to be combined. That is, all of the frames F₀ to F_(N) captured from the time t₀ to the time t_(N) can be used as the images to be combined.

As such, since the frame F_(N/2) which is captured at the time closest to the intermediate time t_(N/2) between the capture time t₀ of the oldest frame F₀ and the capture time t_(N) of the newest frame F_(N) is used as the reference image among a plurality of frames that is stored in the frame memory 12, it is possible to use a large number of frames stored in the frame memory 12 as the images to be combined. Therefore, a good noise reduction effect is expected by the composition of a large number of frames. In particular, since the images to be combined is selected from a group of the frames which are captured earlier than the reference image and a group of the frames which are captured later than the reference image in the frame memory 12, it is possible to use a large number of frames as the images to be combined.

In this embodiment, the frame F_(N/2) captured at the time closest to the time t_(N/2) is used as the reference image. However, the invention is not limited thereto. The reference image may be selected from the frames other than the oldest frame F₀ which is captured at the time t₀ and the newest frame F_(N) which is captured at the time t_(N) in the frame memory 12.

Next, a composite image generation process of the image processing device 10 will be described. FIG. 7 is a flowchart illustrating the composite image generation process of the image processing device 10 in this embodiment. In this embodiment, the frames are input one by one from the outside to the frame memory 12.

(Step S101) The image processing device 10 stores the input frames (video signals) in the frame memory 12. Then, the process proceeds to Step S102.

(Step S102) The composition control unit 11 determines whether a composition instruction is received. When the composition control unit 11 determines that the composition instruction is received, the process proceeds to Step S103. In the other cases, the process returns to Step S101. In the following description, it is assumed that the frame memory 12 stores a plurality of frames.

(Step S103) First, the image selection unit 13 selects the reference image from a group of the frames other than the frame which is captured earliest and the frame F_(N) which is captured latest among a plurality of frames which the image processing device 10 stores in the frame memory 12. In this embodiment, the frame which is captured at the time closest to the intermediate time between the capture time of the oldest frame and the capture time of newest the frame is selected as the reference image. Then, the process proceeds to Step S104.

(Step S104) The image selection unit 13 selects, as a reference image, one frame which is captured at the time closest to the capture time of the frame selected as the reference image from a plurality of frames other than the frame selected as the reference image. The comparative image is images to be combined candidate when the images to be combined are selected. Then, the process proceeds to Step S105. When Step S104 is performed again after Step S106, the image selection unit 13 selects one frame which is captured at the time closest to the capture time of the frame selected as the reference image from the frames which have not been subjected to the determination process in Step S105. In addition, the image selection unit 13 sequentially selects the reference image from the frame which is captured earlier than the reference image and the frame which is captured later than the reference image.

(Step S105) The image selection unit 13 determines whether the frame which is selected as the comparative image in Step S104 can be used as the images to be combined. The image selection unit 13 sets, as the images to be combined, the frame which is determined to be used as the images to be combined. Then, the process proceeds to Step S106. The image selection unit 13 determines whether the selected frame can be used as the images to be combined using, for example, the following method. The image selection unit 13 aligns the position of the object in the reference image and the position of the object in the comparative image. The image selection unit 13 calculates the amount of blur of the object in the comparative image with respect to the object in the reference image. When the calculated amount of blur is less than a threshold value, the image selection unit 13 determines that the selected frame can be used as the images to be combined. The image selection unit 13 set the frame which is determined to be capable of being used as the images to be combined. On the other hand, when the calculated amount of blur is equal to or greater than the threshold value, the image selection unit 13 determines that it is difficult to use the selected frame as the images to be combined. The threshold value is set based on, for example, the allowable range of the region 501 shown in FIG. 4C. In addition, the threshold value may be predetermined or it may be arbitrarily set.

(Step S106) The image selection unit 13 determines whether the determination process in Step S105 has been performed for all frames other than the frame which is selected as the reference image among the frames stored in the frame memory 12. When the image selection unit 13 determines that the process in Step S105 has been performed for all frames other than the frame selected as the reference image, the process proceeds to Step S107. In the other cases, the process returns to Step S104.

(Step S107) The composition processing unit 14 composes all of the images to be combined set in Step S105 with the reference image selected in Step S103 to generate a composite image. Then, the process ends.

As described above, in this embodiment, the image selection unit 13 selects, as the reference image, the frame which is captured at the time closest to the intermediate time between the capture time of the oldest frame and the capture time of the newest frame among the plurality of frames stored in the frame memory 12. In addition, the image selection unit 13 can select the frame as the comparative image from a group of the frames which are captured earlier than the reference image and a group of the frames which are captured later than the reference image. Then, the image selection unit 13 sets, as the images to be combined, the frame which can be used as the images to be combined among the frames which are selected as the comparative images. That is, the image selection unit 13 can select the frame as the images to be combined from a group of the frames which are captured earlier than the reference image and a group of the frames which are captured later than the reference image. Therefore, according to this embodiment, the image processing device 10 can generate a composite image, using a large number of frames which the image processing device 10 stores in the frame memory 12 as the images to be combined. As a result, a good noise reduction effect is expected.

In this embodiment, as a preferred example in which the reference image is selected from the frames other than the oldest frame F₀ which is captured at the time t₀ and the newest frame F_(N) which is captured at the time t_(N) in the frame memory 12, the frame which is captured at the time closest to the intermediate time between the capture time of the oldest frame and the capture time of the newest frame among the plurality of frames stored in the frame memory 12 is selected as the reference image. However, the invention is not limited thereto. For example, the reference image may be selected such that the number of frames which are captured earlier than the frame set as the reference image and are then stored is close to, preferably, equal to the number of frames which are captured later than the frame set as the reference image and are then stored.

In the above-mentioned example, the image selection unit 13 selects all frames (except for the reference image) stored in the frame memory 12, sets the selected frames as the comparative images, and determines whether all of the comparative images can be used as the images to be combined. However, the invention is not limited thereto. For example, the number of images to be combined may be predetermined. In this case, in Step S106, the number of reference frames which are set as the images to be combined is counted and it is determined whether the count result reaches a predetermined number of images to be combined. Even when there is a comparative image which has not been subjected to the process of determining whether the comparative image can be used as the images to be combined, the process may proceed to Step S107 at the time the number of reference frames which are set as the images to be combined reaches a predetermined value.

As such, when the number of images to be combined is predetermined, the comparative image is sequentially selected from the frame which is captured at a time close to the time when the reference image is captured, which is advantageous in improving the efficiency of the composition process in the next stage. The comparative image which is captured at a time close to the time when the reference image is captured is very likely to have a small amount of blur with respect to the reference image. Therefore, the set images to be combined is very likely to be preferentially set from the frame which has a small amount of blur with respect to the reference image. As a result, it is possible to generate a composite image with high efficiency.

Second Embodiment

Next, a second embodiment of the invention will be described with reference to the drawings. In this embodiment, a reference image is selected on the basis of the amount of movement of the object to be estimated by a movement amount estimation unit 21, which will be described below.

FIG. 8 is a block diagram illustrating the structure an image processing device 20 according to this embodiment. In FIG. 8, the image processing device 20 includes a composition control unit 11, a frame memory 12, and the movement amount estimation unit 21. The composition control unit 11 and the frame memory 12 are the same as those in the first embodiment. The movement amount estimation unit 21 estimates the amount of movement X of the object. There are some methods of estimating the amount of movement of the object. For example, the following methods are considered: a method which compares the brightness value of a newly input frame with the brightness value of a previously input frame to estimate the amount of movement of the object in the newly input frame; and a method which detects the amount of movement X of the imaging element 104 using a sensor in an environment in which the object hardly moves and regards the detected amount of movement X of the imaging element as the amount of movement of the object. In this embodiment, the movement amount estimation unit 21 may use any estimation method.

In this embodiment, frame processing has been described. However, when interlaced processing is performed, the amount of movement X of the object may be estimated using a newly input interlace image and an interpolated image based on a previously input interlace image.

Next, a composite image generation process according to this embodiment will be described. FIG. 9 is a flowchart illustrating the composite image generation process of the image processing device 20 in this embodiment. The meaning of the amount of blur is the same as that in the first embodiment and the description thereof will not be repeated.

(Step S201) The image processing device 20 stores frames (video signals) which are input one by one from the outside in the frame memory 12. Then, the process proceeds to Step S202.

(Step S202) The movement amount estimation unit 21 estimates the amount of movement X of the object when the frame which the image processing device 20 stores in the frame memory 12 in Step S201 is captured. Then, the process proceeds to Step S203. As described above, for example, the amount of movement X of the object can be estimated by comparing the frame which is stored in the frame memory 12 in Step S201 with the previous frame.

(Step S203) The image processing device 20 stores the amount of movement X which has been estimated by the movement amount estimation unit 21 in Step S202 so as to be associated with the frame stored in Step S201. Then, the process proceeds to Step S204.

(Step S204) The composition control unit 11 determines whether a composition instruction is received. When the composition control unit 11 determines that the composition instruction is received, the process proceeds to Step S205. In the other cases, the process proceeds to Step S201.

(Step S205) The image selection unit 13 selects the reference image from a plurality of frames that are captured when the amount of movement X of the object is equal to or less than a prescribed amount of movement X_(a) which is a threshold value. In this embodiment, first, the image selection unit 13 excludes a frame F₀ which is captured earliest and a frame F_(N) which is captured latest from a plurality of frames that are captured when the amount of movement X of the object is equal to or less than the prescribed amount of movement X_(a). Next, the image selection unit 13 selects a frame which is captured when the amount of movement X of the object is the minimum among the remaining frames, as the reference image. When there are a plurality of frames satisfying the above-mentioned conditions, a frame which is captured at the time closest to an intermediate (middle) time t_(N/2) between the time t₀ when the oldest frame F₀ is captured and the time t_(N) when the newest frame F_(N) is captured is used as the reference image. Then, the process proceeds to Step S206.

(Step S206) The image selection unit 13 selects one comparative image from a plurality of frames which the image processing device 20 stores in the frame memory 12. Specifically, first, the image selection unit 13 excludes the frame which is selected as the reference image and the frame which has been subjected to the process in Step S207 from the frames. Next, the image selection unit 13 selects a frame which is captured at a time closest to the time when the reference image is captured from the remaining frames. When the estimated amount of movement X of the frame which the image selection unit 13 selects is less than the prescribed amount of movement X_(a) which is a threshold value (which will be described below), the image selection unit 13 selects the selected frame as the comparative image. That is, the images to be combined are selected from a frame group in which the estimated amount of movement X is less than the prescribed amount of movement X_(a). In other words, the image selection unit 13 reads the estimated amount of movement X which is stored so as to be associated with each frame and does not select the frame in which the estimated amount of movement X is equal to or more than the prescribed amount of movement X_(a) as the comparative image, that is, the images to be combined. This process makes it possible to reduce the load of the following process of determining the images to be combined. Then, the process proceeds to Step S207.

(Step S207) The image selection unit 13 determines whether the comparative image selected in Step S206 can be used as the images to be combined. The process of determining whether the comparative image can be used as the images to be combined and the process of setting the images to be combined are the same as those in the first embodiment and the description thereof will not be repeated. Then, the process proceeds to Step S208.

(Step S208) The image selection unit 13 determines whether the process in Step S207 has been performed for all of the frames other than the frame selected as the standard frame and the frame in which the estimated amount of movement X is equal to or less than the prescribed amount of movement X_(a) among a plurality of frames which the image processing device 20 stores in the frame memory 12. When the image selection unit 13 determines that the process in Step S207 has been performed for all of the frames, the process proceeds to Step S208. In the other cases, the process returns to Step S206.

(Step S209) The composition processing unit 14 composes the images to be combined set in Step S207 with the selected reference image to generate a composite image. Then, the process ends.

When the amount of movement X of the object is equal to or more than the prescribed amount of movement X_(a), the amount of blur is greater than an allowable value in the captured image of the object. Therefore, it is preferable that the frame which is captured when the amount of movement X of the object is equal to or more than the prescribed amount of movement X_(a) is not used as the comparative image in terms of the efficiency of the composition process. That is, it is preferable that the image processing device 20 determine whether the comparative image is used as the images to be combined using only the frame which is captured when the amount of movement X of the object is less than the prescribed amount of movement X_(a) in terms of the efficiency of the composition process. In other words, the image processing device 20 selects the images to be combined from the frames which are captured when the amount of movement X of the object is less than the prescribed amount of movement X_(a), which is preferable in terms of the efficiency of the composition process.

Therefore, in this embodiment, since the frame which is captured when the amount of movement X of the object is equal to or more than the prescribed amount of movement X_(a) is not selected as the comparative image, it is possible to reduce the load of the process of determining the images to be combined.

The prescribed amount of movement X_(a), which is a threshold value, may be predetermined or it may be arbitrarily set. For example, when a shutter speed is high, the amount of movement of the object is small even though the speed of the imaging element 104 is high. When the shutter speed is low, the amount of movement of the object is likely to be large even though the speed of the imaging element 104 is low. Therefore, when the shutter speed is high, the prescribed amount of movement X_(a) may be set to a smaller value. When the shutter speed is low, the prescribed amount of movement X_(a) may be set to a larger value. As such, the prescribed amount of movement X_(a) varies depending on the shutter speed when the frame is captured. Therefore, the prescribed amount of movement X_(a) may vary depending on the shutter speed.

It is considered that the movement of the object is caused by the movement of the imaging element and the application of this embodiment depending on the movement of the imaging element 104 will be described using the following reference examples in order to promote the understanding of this embodiment.

First, Reference example 1 in which the imaging element 104 regularly vibrates will be described. FIG. 10 is a schematic diagram illustrating an example of the movement of the imaging element 104. FIG. 11 is a graph illustrating the relationship between the capture time t and the amount of movement X_(N) of the imaging element 104 during the capture of each frame when the imaging element 104 vibrates as shown in FIG. 10. In the example shown in the drawings, the imaging element 104 regularly vibrates from the time t₀ to the time t_(N).

In FIG. 11, the amount of movement X of the imaging element 104 from the time t₀ to the time t_(N) is constantly less than the prescribed amount of movement X_(a). Therefore, the image selection unit 13 determines that all of the frames F₀ to F_(N) captured from the time t₀ to the time t_(N) can be used as the reference image and the comparative image.

Next, a comparative image selection method in Reference example 1 will be described. As described in Step S205 of FIG. 9, first, the image selection unit 13 excludes the frame F₀ which is captured earliest and the frame F_(N) which is captured latest among the frames which are captured when the amount of movement X of the imaging element 104 is equal to or less than the prescribed amount of movement X_(a). Then, the image selection unit 13 selects a frame F_(X) and a frame F_(Z) which are captured when the amount of movement X of the imaging element 104 is the minimum from the remaining frames and sets the selected frames as the candidate frames of the reference image.

As such, when there are a plurality of frames satisfying the comparative image selection conditions, a frame which is captured at the time closest to an intermediate time t_(N/2) between the time t₀ when the oldest frame F₀ is captured and the time t_(N) when the newest frame F_(N) is captured is used as the reference image. Therefore, in Reference example 1, the image selection unit 13 selects the frame F_(Z) as the reference image. A group of the frames other than the reference image (frame F_(N)) among the frames F₀ to F_(N) which are captured from the time t₀ to the time t_(N) is the candidates of the comparative image.

Next, the comparative image selection method will be described. In Reference example 1, as shown in FIG. 11, all of the frames, which are the candidates of the comparative image, are captured when the amount of movement is less than the prescribed amount of movement X_(a). Therefore, all of the frames in the frame memory 12 except for the reference image can be selected as the comparative image.

Next, Reference example 2 in which the imaging element 104 moves irregularly will be described. FIG. 12 is a schematic diagram illustrating an example of the movement of the imaging element 104. FIG. 13 is a graph illustrating the capture time t and the amount of movement X of the imaging element 104 during the capture of each frame when the imaging element 104 moves irregularly as shown in FIG. 12. In FIG. 12, the imaging element 104 moves irregularly for the period from the time t₀ to the time t_(N).

In FIG. 13, there is a frame which is captured when the amount of movement of the imaging element 104 is equal to or more than the prescribed amount of movement X_(a) the frames F₀ to F_(N). For example, a frame F_(Y) is captured when the amount of movement of the imaging element 104 is equal to or more than the prescribed amount of movement X_(a). As shown in FIG. 13, the amount of blur of an object “A” 403 in the frame F_(Y) is large.

For example, a frame F_(X) and a frame F_(Z) are captured when the amount of movement X of the imaging element 104 is less than the prescribed amount of movement X_(a). Therefore, the image selection unit 13 determines that a group of the frames (including the frame F_(X) and the frame F_(Z)) which are captured when the amount of movement X of the imaging element 104 is less than the prescribed amount of movement X_(a) can be used as the reference image and the comparative image.

Next, a comparative image selection method in Reference example 2 will be described. As described in Step S205 of FIG. 9, first, the image selection unit 13 excludes the frame F₀ which is captured earliest and the frame F_(N) which is captured latest among the frames which are captured when the amount of movement X of the imaging element 104 is less than the prescribed amount of movement X_(a). Then, the image selection unit 13 selects a frame F_(X) which is captured when the amount of movement X of the imaging element 104 is the minimum from the remaining frames and sets the selected frame as the reference image.

Next, the comparative image selection method will be described. The image selection unit 13 sequentially selects the comparative image from the frames which are captured earlier than the reference image while determining whether the estimated amount of movement X is less than the prescribed amount of movement X_(a). In Reference example 2, as shown in FIG. 13, a frame in which the estimated amount of movement X is equal to or more than the prescribed amount of movement X_(a) is included in a plurality of frames stored in the frame memory 12. For example, the frame is a frame F_(Y).

The image selection unit 13 reads the estimated amount of movement X which is stored in the frame memory 12 so as to be associated with each frame, does not select the frame in which the estimated amount of movement X is equal to or more than the prescribed amount of movement X_(a), such as the frame F_(Y), as the comparative image, and checks the next frame or another frame. As a result, the image selection unit 13 selects, as the comparative image, only the frame in which the estimated amount of movement X is less than the prescribed amount of movement X_(a). Therefore, in Reference example 2, the number of frames which are selected as the comparative image is less than that in Reference example 1. However, it is obvious that the frame in which the estimated amount of movement X is equal to or more than the prescribed amount of movement X_(a) does not become the images to be combined. As a result, the process of determining whether the frame can be used as the images to be combined causes a reduction in the processing speed or an increase in processing load. Therefore, an inadequate frame is not used for the process of determining whether the frame can be used as the images to be combined and is not selected as the comparative image in a comparative image selection process which is performed before the determination process. As a result, the load of the process of determining whether the frame can be used as the images to be combined and the efficiency of the process is improved.

As described above, in this embodiment, the image selection unit 13 selects the reference image from a plurality of frames which are captured when the amount of movement X of the object is less than the prescribed amount of movement X_(a). Preferably, the frame F₀ which is captured earliest and the frame F_(N) which is captured latest are excluded from a plurality of frames which are captured when the amount of movement X of the object is less than the prescribed amount of movement X_(a) and a frame which is captured when the amount of movement X of the object is the minimum among the remaining frames is selected as the reference image. If there are a plurality of frames which are captured when the amount of movement X of the object is the minimum, the image selection unit 13 selects, as the reference image, a frame which is captured at the time closest to the intermediate time t_(N/2) between the time t₀ when the oldest frame F₀ is captured and the time t_(N) when the newest frame F_(N) is captured.

In addition, the image selection unit 13 selects as the comparative image, only the frame which is captured when the amount of movement X of the object is less than the prescribed amount of movement X_(a) from the frames which are captured earlier and later with respect to the reference image. That is, the frame which is obviously improper as the images to be combined and is more than the prescribed amount of movement X_(a) and is more equal to or more than is not used for the process of determining whether the frame can be used as the images to be combined. In other words, the image selection unit 13 does not select the frame which is obviously improper as the images to be combined. Therefore, it is possible to reduce the load of the process.

In the above-mentioned example, the image selection unit 13 determines whether all of the frames which are selected as the comparative image can be used as the images to be combined. However, the invention is not limited thereto. For example, a predetermined number of frames may be set in advance and the process may proceed to Step S209 even when there is a comparative image which has not been determined at the time the number of frames which are set as the images to be combined reaches the predetermined number of frames. In particular, in this case, the process of determining whether the frame can be used as the images to be combined is preferentially performed for the comparative image which is captured at the time close to the capture time of the reference image, which makes it possible to improve the efficiency of the composite image generation process.

The known technique may be used as the method of estimating the amount of movement of the object. For example, the movement of the object may be estimated by comparing the brightness values of two consecutive frames or by detecting the amount of movement of the imaging element using a sensor. In addition, the amount of movement of the object may be directly calculated or an element including another amount of movement which substitutes the amount of movement of the object may be calculated. For example, when the capture interval of the frame is predetermined, a speed, that is, the speed of the object or the imaging element may be calculated instead of the amount of movement.

Third Embodiment

Next, a third embodiment of the invention will be described with reference to the drawings. In this embodiment, the minimum total number of necessary comparative images and images to be combined (hereinafter, referred to as the number of images required for composition) is set on the basis of the amount of noise included in each frame.

FIG. 14 is a block diagram illustrating the structure of an image processing device 30 according to this embodiment. In the example shown in FIG. 14, the image processing device 30 includes a composition control unit 11, a frame memory 12, and a noise amount estimation unit 31. The composition control unit 11 and the frame memory 12 are the same as those in the second embodiment.

The noise amount estimation unit 31 estimates the amount of noise in the frame. For example, the amount of noise is estimated on the basis of a gain value. The noise amount estimation unit 31 uses the gain value which is determined by, for example, AE control. In general, as the gain value increases, the amount of noise increases. As the gain value is reduced, the amount of noise is reduced. For example, when the gain value is large, the amount of noise estimated by the noise amount estimation unit 31 is large. When the gain value is small, the amount of noise estimated by the noise amount estimation unit 31 is small. Any method may be used as long as it can estimate the amount of noise in the frame.

In general, as the number of frames to be composed (images to be combined) increases, the amount of noise to be removed increases. Therefore, in this embodiment, when the amount of noise estimated by the noise amount estimation unit 31 is large, the image selection unit 13 increases the number of images to be combined to a large value. When the amount of noise estimated by the noise amount estimation unit 31 is small, the image selection unit 13 sets the number of images to be combined to a small value. Hereinafter, the sum of the number of images to be combined which are required in accordance with the amount of noise and the number of reference images (one reference image) is referred to as the number of images K required for composition.

Next, a composite image generation process in this embodiment will be described. FIG. 15 is a flowchart illustrating the composite image generation process of the image processing device 30 according to this embodiment.

The description of the same content as that in the first and second embodiments will not be repeated.

(Step S301) The image processing device 10 stores frames (video signals) which are input one by one from the outside in the frame memory 12.

(Step S302) The noise amount estimation unit 31 estimates the amount of noise in each frame which the image processing device 10 stores in the frame memory 12. Then, the process proceeds to Step S303.

(Step S303) The image processing device 10 stores the amount of noise which is estimated by the noise amount estimation unit 31 in Step S302 so as to be associated with each frame stored in Step S301, in the frame memory 12. Then, the process proceeds to Step S304.

(Step S304) The composition control unit 11 determines whether a composition instruction is received. When the composition control unit 11 determines that a composition instruction is received, the process proceeds to Step S305. In the other cases, the process returns to Step S301.

(Step S305) The noise amount estimation unit 31 calculates the average value of the amounts of noise of the frames stored in the frame memory 12 on the basis of the amount of noise of each frame stored in the frame memory 12. Then, the process proceeds to Step S306.

(Step S306) The image selection unit 13 calculates the number of images K required for composition on the basis of the average value of the amounts of noise calculated in Step S305. Then, the process proceeds to Step S307.

(Step S307) The image selection unit 13 selects, as the reference image, the frame which is captured at the time closest to the intermediate time between the time when the oldest frame is captured and the time when the newest frame is captured among a plurality of frames which the image processing device 10 store in the frame memory 12.

This process is the same as that in Step S103 of FIG. 7. Then, the process proceeds to Step S308.

(Step S308) The image selection unit 13 selects one comparative image from the plurality of frames which the image processing device 10 store in the frame memory 12. Specifically, the comparative image is preferentially selected from the frames which are captured at the time closest to the capture time of the reference image, among the frames other than the frame which is selected as the comparative image and the frame which has been subjected to the process in Step S309. This process is the same as that in Step S206 of FIG. 9. That is, the images to be combined is preferentially selected from the frames which are captured at the time closest to the capture time of the reference image, which will be described below. Then, the process proceeds to Step S309.

(Step S309) The image selection unit 13 determines whether the comparative image selected in Step S308 can be used as the images to be combined. Since the determination method is the same as that in Step S105 of FIG. 7, the description thereof will not be repeated in this embodiment. The image selection unit 13 sets the comparative image which is determined to be used as the images to be combined as the images to be combined.

Then, the process proceeds to Step S310.

(Step S310) The image selection unit 13 determines whether the sum of the number of reference images and the number of set images to be combined is equal to the number of images K required for composition which is calculated in Step S306. When the image selection unit 13 determines that the sum is “equal to” the number of images K required for composition, the process of determining whether the comparative image can be used as the images to be combined ends even though there is a frame which has not been selected as the comparative image, in the frame memory 12. Then, the process proceeds to Step S311. When the image selection unit 13 determines that the sum is “not equal to the number of images K required for composition (the sum does not reach the number of images K required for composition)”, the process returns to Step S308 and Steps S308 and S309 are repeatedly performed until the sum of the number of reference images and the number of set images to be combined reaches the number of images K required for composition.

(Step S311) The composition processing unit 14 composes the ((K−1)) images to be combined selected in Step S309 with the reference image selected in Step S307 to generate a composite image. Then, the process ends.

As described above, according to this embodiment, the image selection unit 13 selects, as the reference image, the frame which is captured at the time closest to the intermediate time between the time when the oldest frame is captured and the time when the newest frame is captured among a plurality of frames which the image processing device 10 store in the frame memory 12.

In addition, the noise amount estimation unit 31 estimates the amount of noise in each frame and calculates the average value of the amounts of noise.

The image selection unit 13 determines the number of frames (the number of images K required for composition) used to generate a composite image, on the basis of the average value of the amounts of noise calculated by the noise amount estimation unit 31. The composition process is performed using only the number of images K required for composition. Therefore, since the composition processing unit 14 generates a composite image using only the number of frames corresponding to the amount of noise, that is, only the minimum number of necessary frames, it is possible to reduce the load of the composition process.

The images to be combined selected by the image selection unit 13 is preferentially selected from the frames which are captured at the time close to the capture time of the reference image and have a relatively small amount of blur. Therefore, it is possible to improve the efficiency of the composite image generation process.

Fourth Embodiment

Next, a fourth embodiment of the invention will be described with reference to the drawings. In this embodiment, before a reference image is selected, a frame section including a plurality of frames (the number of frames which is less than the number of frames stored in a frame memory) is set on the basis of the estimated amount of movement X of an object. In other words, a reference image selection frame section including a plurality of frames which are consecutive in order of capture time is set on the basis of the estimated amount of movement X of the object and the reference image is selected from a plurality of frames forming the reference image selection frame section.

In this embodiment, the number of frames forming the frame section is described as the number of images K required for composition and the reference image selection frame section is described as a frame section in which the integrated value of the estimated amount of movement is the minimum.

FIG. 16 is a block diagram illustrating the structure of an image processing device 40 according to this embodiment. In FIG. 16, the image processing device 40 includes a composition control unit 11, a frame memory 12, a movement amount estimation unit 21, and a noise amount estimation unit 31. The composition control unit 11, the frame memory 12, and the movement amount estimation unit 21 are the same as those in the second embodiment. The noise amount estimation unit 31 is the same as the noise amount estimation unit 31 in the third embodiment.

FIG. 17 is a flowchart illustrating a composite image generation process of the image processing device 40 in this embodiment. The description of the same content as that described in the first to third embodiments will not be repeated.

(Step S401) The image processing device 10 stores frames (video signals) which are input one by one from the outside in the frame memory 12.

(Steps S402 and S403) The movement amount estimation unit 21 estimates the amount of movement X of the object when each frame which is stored in frame memory 12 in Step S401 is captured. Then, the frame memory 12 stores the estimated amount of movement X so as to be associated with each frame. This process is the same as Steps S202 and S203 in FIG. 9.

(Step S404, S405) The noise amount estimation unit 31 estimates the amount of noise of each frame stored in the frame memory 12. Then, the frame memory 12 stores the estimated amount of noise so as to be associated with each frame. This process is the same as Steps S302 and S303 in FIG. 15.

(Step S406) The composition control unit 11 determines whether a composition instruction is received. When the composition control unit 11 determines that a composition instruction is received, the process proceeds to Step S407. In the other cases, the process returns to Step S401.

(Step S407) The noise amount estimation unit 31 calculates the average value of the amounts of noise of the frames stored in the frame memory 12 on the basis of the amount of noise of each frame stored in the frame memory 12. Then, the process proceeds to Step S408.

(Step S408) The image selection unit 13 calculates the minimum number of necessary frames required to generate a composite image, that is, the number of images K required for composition, which is the sum of the number of reference images and the number of images to be combined, on the basis of the average value of the amounts of noise which is calculated by the noise amount estimation unit 31 in Step S407. Then, the process proceeds to Step S409. This process is the same as Step S306 in FIG. 15.

(Step S409) When selecting the reference image, first, the image selection unit 13 specifies a frame section including K frames in which the amount of movement X is less than a prescribed amount of movement X_(a), which is a threshold value, from a plurality of frames stored in the frame memory 12. The frame section is a plurality of frame groups having the oldest frame at the head. Then, the first frame in the frame section is shifted backward one by one to sequentially specify a plurality of frame sections. This process will be described with reference to FIG. 18.

FIG. 18 is a graph illustrating an example of the relationship between a time t and the amount of movement X of the imaging element 104 in each frame stored in the frame memory 12. In the graph, a circle indicates a frame, a black circle indicates a frame in which the amount of movement X is less than the prescribed amount of movement X_(a), and a white circle indicates a frame in which the amount of movement X is equal to or more than the prescribed amount of movement X_(a).

First, the image selection unit 13 specifies a frame section T₀ in which the first frame is a frame F₀ which is captured at the earliest time t₀. The image selection unit 13 reads the estimated amount of movement X of the frame F₀, which is the first frame, from the frame memory 12 and determines whether the estimated amount of movement X is less than the prescribed amount of movement X_(a). When the estimated amount of movement X is less than the prescribed amount of movement X_(a), the image selection unit 13 counts the frame F₀ as the frame in which the estimated amount of movement X is less than the prescribed amount of movement X_(a). When the determination process for the frame F₀ ends, the image selection unit 13 determines whether the estimated amount of movement X is less than the prescribed amount of movement X_(a) for the next frame F₁ which is captured after the frame F₀. When the estimated amount of movement X is less than the prescribed amount of movement X_(a), the image selection unit 13 counts the frame F₁ as the frame in which the estimated amount of movement X is less than the prescribed amount of movement X_(a). This process is repeatedly performed until the counted number of frames reaches the number of images K required for composition. When the count result reaches the number of images K required for composition, a K-th frame is the last frame of the frame section T₀. In this way, the frame section T₀ is specified. FIG. 18 shows an example in which K is 9 and the frame section T₀ is from the frame F₀ to a frame F₁₁.

When the frame section T₀ is specified, the image selection unit 13 specifies the next frame section. The image selection unit 13 proceeds to a process of shifting the first frame to the next captured frame F₁ of frame F₀ and specifying process frame section T₁ having the frame F₁ as the first frame. Then, similarly to the operation of specifying the frame section T₀, the image selection unit 13 searches for a frame in which the amount of movement X is less than the prescribed amount of movement X_(a), counts the number of frames, sets the frame as the last frame in the frame section at the time the count result reaches the number of images K required for composition, and specifies the frame section T₁. The image selection unit 13 repeatedly performs the specifying process of this frame section until the frame F_(N) which is captured latest among the frames stored in the frame memory 12 becomes the last frame of the frame section. Then, the image selection unit 13 does not set, as the frame section, a frame section that does not include K frames in which the amount of movement X is less than the prescribed amount of movement X_(a) among the frame sections. The above-mentioned process is performed to specify the frame sections T₀, T₁, T₂, T₃ . . . .

(Step S410) Then, the image selection unit 13 specifies the reference image selection frame section in which the integrated value of the amount of movement X of the object is the minimum from the specified frame sections T₀, T₁, T₂, T₃ . . . . The image selection unit 13 calculates the integrated value of the estimated amounts of movement for each frame section, on the basis of each frame in which the amount of movement X is less than the prescribed amount of movement X_(a) in each frame section. Then, the image selection unit 13 performs the calculation process for all of the frame sections T₀, T₁, T₂, T₃ . . . which are obtained in Step S409. Then, the image selection unit 13 specifies, as the reference image selection frame section, the frame section in which the calculated integrated value of the amounts of movement is the minimum.

FIG. 19 is a graph illustrating only the frame section T₀ having the frame F₀ as the first frame and the frame section T_(s) having the frame F_(s) as the first frame among a plurality of frame sections.

The amount of the integration of the amount of movement in the frame section T_(s) is less than the amount of integration of the amount of movement in the frame section T₀. In this case, the image selection unit 13 specifies the frame section T_(s) as the reference image selection frame section.

(Step S411) The image selection unit 13 selects the reference image from a plurality of frames included in the specified reference image selection frame section. Specifically, first, the image selection unit 13 excludes the frame which is captured earliest and the frame which is captured latest in the reference image selection frame section from the candidates of the reference image. Then, the image selection unit 13 selects the frame which is captured when the estimated amount of movement X of the object is the minimum from the remaining frames, as the reference image.

This process will be described with reference to FIG. 20. In the graph shown in FIG. 20, a frame is indicated by a black circle in the reference image selection frame section T_(s) shown in FIG. 19. First, the image selection unit 13 excludes the first frame F_(s) and the last frame F_(t) in the reference image selection frame section T_(s) from the comparative image candidates. Then, the image selection unit 13 reads the estimated amount of movement relative to the remaining frames in the reference image selection frame section T_(s) from the frame memory 12 and searches for the frame in which the estimated amount of movement is the minimum. Then, the image selection unit 13 selects, as the reference image, a frame F_(m) which is captured at a time t_(m).

(Step S412) After the reference image selection process ends, the image selection unit 13 proceeds to a comparative image selection process. First, the image selection unit 13 excludes the standard frame and the frame which has been subjected to the process in Step S413, which will be described below, from a plurality of frames stored in the frame memory 12. Then, the image selection unit 13 selects, from the remaining frames, the frame which is captured at the time closest to the capture time of the reference image as a comparative image candidate. Then, when the amount of movement X is less than the prescribed amount of movement X_(a) in the frame which is selected as the comparative image candidate, the image selection unit 13 selects the selected frame as the comparative image. When the amount of movement is equal to or more than the prescribed amount of movement X_(a) in the frame which is selected as the comparative image candidate, the image selection unit 13 determines that the frame is not the comparative image and selects the next frame which is captured at the time close to the capture time of the reference image as the comparative image candidate. This process is the same as Step S206 in FIG. 9.

The image selection unit 13 may end the comparative image selection process when the frame selected as the comparative image candidate is determined not to be the comparative image, the next selected comparative image candidate is determined not to be the comparative image, and the comparative image candidate is determined not to be the comparative image a prescribed consecutive number of times as a result and proceed to Step S415. The prescribed number of times may be predetermined or it may be arbitrarily set.

In this embodiment, the selected comparative image is preferentially selected from the frames in the previously obtained reference image selection frame section. However, the comparative image may be selected from the frames outside the reference image selection frame section. The image selection unit 13 selects all of the frames in the reference image selection frame section as the comparative image and determines whether the comparative image is used as the images to be combined. As a result, when the number of frames which are set as the images to be combined does not reach the number of images K required for composition including the reference image, the image selection unit 13 selects, as the comparative image, the frame which is outside the reference image selection frame section, which is captured at the time closest to the capture time of the reference image, and of which the amount of movement is less than the prescribed amount of movement X_(a). Then, the process proceeds to Step S413.

(Step S413) The image selection unit 13 determines whether the comparative image selected in Step S412 can be used as the images to be combined. When it is determined that the selected comparative image can be used as the images to be combined, the image selection unit 13 sets the frame which is selected as the comparative image as the images to be combined. The determination method is the same as that in Step S207 in FIG. 9. Then, the process proceeds to Step S414.

When it is determined a prescribed consecutive number of times that the frame selected as the comparative image is not available as the images to be combined, the image selection unit 13 may end the process of determining whether the comparative image can be used as the images to be combined and proceed to Step S415. The prescribed number of times may be predetermined or it may be arbitrarily set.

(Step S414) The image selection unit 13 determines whether the sum of the number of a reference image (single) and the number of set images to be combined is equal to the number of images K required for composition calculated in Step S408. When the image selection unit 13 determines that the sum is “equal to” the number of images K required for composition, the process proceeds to Step S415. In the other cases, the process returns to Step S412 and Steps S412 and S413 are repeatedly performed until the sum of the number of reference images and the number of set images to be combined reaches the number of images K required for composition.

(Step S415) The composition processing unit 14 composes ((K−1)) frames which are set as the images to be combined with the reference image to generate a composite image.

As described above, according to this embodiment, the reference image is selected from a plurality of frames in which the amount of movement of the object is the minimum in the frame section (reference image selection frame section) and the images to be combined is preferentially set from the frames which are captured at the time close to the capture time of the reference image. Therefore, it is possible to preferentially set the images to be combined with a small amount of blur or a small amount of movement relative to the reference image. As a result, it is possible to generate a composite image using the frames which are suitable for composition and thus to improve the efficiency of the composition processing.

Fifth Embodiment

Next, a fifth embodiment of the invention will be described with reference to the drawings. FIG. 21 is a block diagram illustrating the structure of an endoscope apparatus according to this embodiment. In the example shown in FIG. 21, an endoscope apparatus 101 includes an elongated insertion unit 102, a main body unit 103, and a liquid crystal display (LCD) 130. In addition, a recording medium 132 can be attached to the main body unit 103. For example, still images or moving images which are captured by the endoscope apparatus 101 can be recorded on the recording medium 132.

The insertion unit 102 includes an objective lens 127, an imaging element (charge coupled device (CCD)) 104, a light emitting diode (LED) 106, and a wire fixing unit 119. The objective lens 127, the imaging element 104, the LED 106, and the wire fixing unit 119 are arranged at a tip 128 (endoscope tip) of the insertion unit 102.

The main body unit 103 includes an imaging element drive circuit 105, a pre-amplifier 131, image processing means 107, an analog front end (AFE) 109, system control means 110, a user interface 111, an LED drive circuit 113, UD (up/down) curvature motor 120, an RL (right/left) curvature motor 121, curvature control means 122, image recording means 115, and an LCD controller 117. In addition, the endoscope apparatus 101 includes four wires 123 to 126 for curving the tip 128 of the insertion unit 102 in all directions.

The user interface 111 includes, for example, a switch or a joystick for curving the tip of the endoscope, receives an instruction input from the user, and outputs a signal corresponding to the received input to the system control means 110. Examples of the instruction from the user include an instruction for a zoom ratio, an instruction for the brightness of the image to be captured, an instruction to turn on and off the LED 106, an instruction to curve the tip 128 of the insertion unit 102, an instruction to record images on the recording medium 132, and an instruction to display images o the LCD 130.

The system control means 110 controls each unit of the endoscope apparatus 101 such that a process based on the signal input from the user interface 111 is performed. For example, when the user interface 111 receives an input indicating the zoom ratio or an input indicating the brightness of the image to be captured, the system control means 110 controls the image processing means 107 such that a process based on the input received by the user interface 111 is performed. In addition, when the user interface 111 receives an input indicating the turn-on and turn-off of the LED 106, the system control means 110 controls the LED drive circuit 113 such that a process based on the input received by the user interface 111 is performed. When the user interface 111 receives an input indicating the curving of the tip 128 of the insertion unit 102, the system control means 110 controls the curvature control means 122 such that a process based on the input received by the user interface 111 is performed. When the user interface 111 receives an input indicating the recording of images on the recording medium 132 or an input indicating the display of images on the LCD 130, the system control means 110 controls the image recording means 115 such that a process based on the input received by the user interface 111 is performed.

The wire fixing unit 119 fixes the four wires 123 to 126 for curving the tip 128 of the insertion unit 102 in all directions. Among the four wires 123 to 126, two wires 123 and 124 for curving the tip 128 of the insertion unit 102 in the up-down direction are connected to the UD curvature motor 120. In addition, two wires 125 and 126 for curving the tip 128 of the insertion unit 102 in the left-right direction are connected to the RL curvature motor 121.

The UD curvature motor 120 and the RL curvature motor 121 are connected to the curvature control means 122. The curvature control means 122 is connected to the system control means 110. The curvature control means 122 controls the driving of the UD curvature motor 120 on the basis of a curvature instruction signal related to the up-down direction which is input from the system control means 110 and controls the driving of the RL curvature motor 121 on the basis of a curvature instruction signal related to the left-right direction which is input from the system control means 110. The UD curvature motor 120 draws the two wires 123 and 124 to curve the tip 128 of the insertion unit 102 in the up-down direction under the control of the curvature control means 122. In addition, the RL curvature motor 121 draws the two wires 125 and 126 to curve the tip 128 of the insertion unit 102 in the left-right direction under the control of the curvature control means 122.

According to this structure, when the user tilts the joystick of the user interface 111 in the up-down direction, the system control means 110 transmits the curvature instruction signal related to the up-down direction to the curvature control means 122. The curvature control means 122 controls the driving of the UD curvature motor 120 on the basis of the received curvature instruction signal and draws the wires 123 and 124 connected to the UD curvature motor 120. In this way, the endoscope apparatus 101 can curve the tip 128 of the insertion unit 102 in the up-down direction. The tip is curved in the left-right direction by the same method as described above. When the user tilts the joystick in the left-right direction, the system control means 110 transmits the curvature instruction signal related to the left-right direction to the curvature control means 122. The curvature control means 122 controls the driving of the RL curvature motor 121 on the basis of the received curvature instruction signal and draws the wires connected to the RL curvature motor 121. In this way, the endoscope apparatus 101 can curve the tip 128 of the insertion unit 102 in the left-right direction.

The LED 106 is connected to the LED drive circuit 113 through a cable. The LED drive circuit 113 is connected to the system control means 110. The LED drive circuit 113 controls the turn-on and turn-off of the LED 106 on the basis of an LED turn-on signal input from the system control means 110. The LED 106 is turned on and off under the control of the LED drive circuit 113.

The objective lens 127 forms an object image illustrated by the LED 106 on a light receiving surface of the imaging element 104. The imaging element 104 is connected to the imaging element drive circuit 105 and the pre-amplifier 131 through a combined coaxial cable. The imaging element drive circuit 105 receives a timing signal for driving the imaging element 104 from a timing generator which is provided in the image processing means 107. Then, the imaging element drive circuit 105 performs a drive process corresponding to the length of a transmission path (the length of the combined coaxial cable) to the imaging element 104 for the received timing signal and transmits the processed timing signal as an imaging element driving signal to the imaging element 104.

The imaging element 104 converts the light focused on the light receiving surface into an electric signal on the basis of the timing of the transmitted imaging element driving signal and outputs the converted electric signal as an imaging element output signal. The imaging element output signal output from the imaging element 104 is input to the pre-amplifier 131 through the combined coaxial cable. The pre-amplifier 131 amplifies the imaging element output signal in order to compensate for a signal level which is attenuated by transmission through the combined coaxial cable. The imaging element output signal amplified by the pre-amplifier 131 is input to the AFE 109. The AFE 109 performs a correlated double sampling (CDS) process, an auto gain control (AGC) process, or an analog/digital (AD) conversion process for the input imaging element output signal. The imaging element output signal (frame) processed by the AFE 109 is input to the image processing means 107.

The image processing means 107 includes at least one of the image processing device 10 according to the first embodiment, the image processing device 20 according to the second embodiment, the image processing device 30 according to the third embodiment, and the image processing device 40 according to the fourth embodiment. The image processing means 107 generates a composite image using the input imaging element output signal (frame) and removes noise, under the control of the system control means 110. The image processing means 107 according to this embodiment generates the composite image using any one of a plurality of generation methods according to the first to fourth embodiments, according to the image processing devices 10, 20, 30, and 40 provided therein.

The image processing means 107 may perform various kinds of camera signal processing, such as white balance correction, gamma correction, contour correction, electronic zoom processing, color correction, contrast correction, and AE control. In addition, the image processing means 107 outputs the generated composite image as image data to the image recording means 115.

The image recording means 115 outputs the input image data to the LCD controller 117. In addition, the image recording means 115 performs a freeze process of displaying a still image on the LCD 130. The image recording means 115 compresses the input image data with an encoder and records the image data as still image data or moving image data on the recording medium 132. This image recording operation is performed on the basis of the input from the user interface 111 when the system control means 110 transmits a recording signal to the image recording means 115 and the image recording means 115 receives the recording signal.

The endoscope apparatus 101 captures a still image when image recording is performed after the image recording means 115 performs the freeze process and captures a moving image when image recording is performed without the freeze process. The image recording means 115 performs an image reproduction operation of reading still image data or moving image data from the recording medium 132, decompressing the data, and outputting the decompressed data to the LCD controller 117. This image reproduction operation is performed when the system control means 110 receives an image display signal from the user interface 111.

The LCD controller 117 performs image processing (for example, gamma correction, scaling, and RGB conversion) which is most suitable for the connected LCD 130 for various kinds of image data input from the image recording means 115 and outputs the processed image data to the LCD 130. The LCD 130 displays an image on the basis of the input image data.

As described above, the image processing means 107 of the endoscope apparatus 101 includes at least one of the image processing device 10 according to the first embodiment, the image processing device 20 according to the second embodiment, the image processing device 30 according to the third embodiment, and the image processing device 40 according to the fourth embodiment. Therefore, the image processing means 107 can generate a composite image using the frame which is more suitable for composition among the frames captured by the imaging element 104 of the endoscope apparatus 101. In addition, the image processing means 107 can obtain the same effect as the image processing devices 10, 20, 30, and 40, according to the image processing devices 10, 20, 30, and 40 provided therein.

A plurality of embodiments of the invention have been described above with reference to the drawings. However, the detailed structure is not limited to the plurality of embodiments and the invention also includes, for example, the structure which is designed without departing from the scope and spirit of the invention.

All or some of the functions of each unit provided in the image processing devices 10, 20, 30, and 40 according to the above-described embodiments may be implemented by recording a program for implementing the functions on a computer-readable recording medium and allowing a computer system to reading the program recorded on the recording medium and to execute the program. The term “computer system” includes an OS and hardware, such as peripheral devices.

Typically, examples of the “computer-readable recording medium” include portable media, such as a flexible disk, a magneto-optical disk, a ROM, and a CD-ROM, and a recording unit, such as a hard disk provided in the computer system. However, the computer-readable recording medium is not necessarily limited thereto.

In addition, the following means may be used instead of the “computer-readable recording medium”. For example, the computer-readable recording medium may be a medium that dynamically stores a program for a short period of time, such as a communication line used when a program is transmitted through a network such as the Internet or a communication line such as a telephone line. In addition, in this case, the computer-readable recording medium may be a medium that stores a program for a predetermined period of time, such as a volatile memory provided in a computer system serving as a server or client. Furthermore, the program may be executed to implement some of the above-mentioned functions. Further, the above-mentioned functions may be implemented by combinations of all programs recorded on the computer system.

While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims. 

What is claimed is:
 1. An image processing device comprising: a memory configured to store a plurality of captured images; and a processor comprising hardware, wherein the processor is configured to implement: an image selection unit configured to: select candidate images from among the plurality of captured images by excluding an earliest image which is captured earliest among the plurality of captured images and by excluding a latest image which is captured latest among the plurality of captured images; select a reference image from among the candidate images; and select images to be combined with the reference image from: a first group of images among the plurality of captured images, the first group of images being captured earlier than the reference image; and a second group of images among the plurality of captured images, the second group of images being captured later than the reference image; and a composition processing unit configured to compose the images to be combined selected by the image selection unit with the reference image selected by the image selection unit to generate a composite image.
 2. The image processing device according to claim 1, wherein the image selection unit is configured to select the images to be combined with the reference image by: selecting images from the first group of images and the second group of images in order from a closest capture time to a capture time of the reference image to a farthest capture time to the capture time of the reference image, and determining whether the selected images can be used as the images to be combined with the reference image in the order in which the images from the first group of images and the second group of images are selected.
 3. The image processing device according to claim 1, wherein the image selection unit is configured to select, as the reference image, an image which is captured at a time closest to an intermediate time between a time when the earliest image is captured and a time when the latest image is captured among the candidate images.
 4. The image processing device according to claim 1, wherein the image selection unit is configured to select the reference image such that the number of images which are captured earlier than the reference image is closest to the number of images which are captured later than the reference image, among the candidate images.
 5. The image processing device according to claim 1, wherein the processor is further configured to implement: a movement amount estimation unit configured to estimate an amount of movement of an object in one or more images among the plurality of captured images, wherein the image selection unit is configured to select the reference image based on the amount of movement estimated by the movement amount estimation unit.
 6. The image processing device according to claim 5, wherein the movement amount estimation unit is further configured to compare a plurality of images among the plurality of captured images to estimate the amount of movement of the object in the plurality of images.
 7. The image processing device according to claim 5, wherein the movement amount estimation unit is further configured to detect the amount of movement of an imaging sensor that captures the plurality of captured images to estimate the amount of movement of the object in the plurality of images.
 8. The image processing device according to claim 5, wherein the image selection unit is further configured to select the reference image from a plurality of images in which the amount of movement is equal to or less than a predetermined threshold value.
 9. The image processing device according to claim 5, wherein the image selection unit is further configured to select the candidate images from among the plurality of captured images by excluding an image which is captured earliest and an image which is captured latest among a plurality of images in which the amount of movement is equal to or less than a predetermined threshold value.
 10. The image processing device according to claim 5, wherein the image selection unit is configured to select the reference image from an image section including a predetermined number of images which are consecutive in order of capture time and are less than the number of the plurality of captured images stored in the memory, and wherein in the image section, an integrated value of the amount of movement in a plurality of images forming the image section among the plurality of captured image is the minimum.
 11. The image processing device according to claim 10, wherein the image selection unit is further configured to exclude, from candidates of the reference image, the image which is captured earliest and the image which is captured latest among the plurality of images forming the image section among the plurality of captured images.
 12. The image processing device according to claim 5, wherein the image selection unit is further configured to select an image among the candidate images in which the estimated amount of movement is the minimum as the reference image.
 13. The image processing device according to claim 1, wherein the image selection unit is further configured to: determine whether a predetermined number of consecutive images are not selected as the images to be combined with the reference image; and end the selecting of images to be combined with the reference image when it is determined that the predetermined number of consecutive images are not selected as the images to be combined with the reference image.
 14. The image processing device according to claim 1, wherein the processor is further configured to implement: a noise amount estimation unit configured to estimate an amount of noise in the plurality of captured images stored in the memory, wherein the image selection unit is further configured to determine the number of images required to be combined with the reference image to generate the composite image based on the amount of noise estimated by the noise amount estimation unit.
 15. The image processing device according to claim 14, wherein the image selection unit is further configured to: count the number of the images to be combined with the reference image; and end selecting the images to be combined with the reference image when the number of counted images reaches the number of images required to be combined with the reference image to generate the composite image.
 16. The image processing device according to claim 1, wherein the image selection unit is configured to: predetermine the number of the images to be combined with the reference image; repeat a process of: selecting an image to be combined with the reference image from the first group of images and the second group of images; and determining whether the number of selected images to be combined with the reference image has reached the predetermined number of the images to be combined with the reference image; and end the process of selecting the image to be combined with the reference image and determining whether the number of selected images to be combined has reached the predetermined number of images to be combined when the number of selected the images to be combined reaches the predetermined number of the images to be combined.
 17. The image processing device according to claim 5, wherein the image selection unit is further configured to select the images to be combined with the reference image from a plurality of images in which the amount of movement estimated by the movement amount estimation unit is equal to or less than a predetermined threshold value.
 18. An image processing method comprising: selecting candidate images from among a plurality of captured images by excluding an earliest image which is captured earliest among the plurality of captured images and by excluding a latest image which is captured latest among the plurality of captured images; selecting a reference image from among the candidate images; selecting images to be combined with the reference image from: a first group of images among the plurality of captured images, the first group of images being captured earlier than the reference image; and a second group of images among the plurality of captured images, the second group of images being captured later than the reference image; and composing the images to be combined with the selected reference image to generate a composite image.
 19. The image processing method according to claim 18, wherein selecting the images to be combined with the reference image comprises: selecting images from the first group of images and the second group of images in order from a closest capture time to a capture time of the reference image to a farthest capture time to the capture time of the reference image, and determining whether the selected images can be used as the images to be combined with the reference image in the order in which the images from the first group of images and the second group of images are selected.
 20. The image processing method according to claim 18, wherein selecting the reference image from among the candidate images comprises selecting an image from among the candidate images which is captured at a time closest to an intermediate time between a time when the earliest image is captured and a time when the latest image is captured as the reference image.
 21. The image processing method according to claim 18, wherein selecting the reference image from among the candidate images comprises selecting the reference image such that the number of images which are captured earlier than the reference image is closest to the number of images which are captured later than the reference image, among the plurality of captured images stored in the memory.
 22. The image processing method according to claim 18, further comprising: for images among the candidate images: comparing a main image with another image among the candidate images; and estimating an amount of movement of an object in the main image based on a result of the comparison of the main image with the another image among the candidate images, wherein selecting the reference image comprises selecting the reference image from among the candidate images image based on the estimated amount of movement each of the images among the candidate images.
 23. The image processing method according to claim 22, wherein selecting the reference image comprises selecting an image in which the estimated amount of movement as the reference image.
 24. The image processing method according to claim 22, wherein selecting the images to be combined comprises selecting the images to be combined from a plurality of images in which the estimated amount of movement is equal to or less than a predetermined threshold value.
 25. A computer-readable device storing a program causing a computer to perform steps of: selecting candidate images from among a plurality of captured images by excluding an earliest image which is captured earliest among the plurality of captured images and by excluding a latest image which is captured latest among the plurality of captured images; selecting a reference image from among the candidate images; selecting images to be combined with the reference image from: a first group of images among the plurality of captured images, the first group of images being captured earlier than the reference image; and a second group of images among the plurality of captured images, the second group of images being captured later than the reference image; and composing the images to be combined with the reference image to generate a composite image. 